1. Field of the Disclosure
The present disclosure relates to a liquid crystal display device, and more particularly, to a liquid crystal display device with wide-viewing angles.
2. Discussion of the Related Art
With rapid development of information technologies, various types of display devices for displaying images have been required. Recently, flat panel display (FPD) devices such as liquid crystal display (LCD) devices, plasma display panel (PDP) devices and organic electroluminescent display (OLED) devices have been widely used.
Among the various types of FPD devices, liquid crystal display (LCD) devices have been widely used as monitors for notebook computers and desktop computers because of their excellent contrast ratio, low power consumption and superiority in displaying moving images.
Generally, an LCD device includes two substrates and liquid crystal material therebetween. The two substrates include respective electrodes and are disposed such that the electrodes face each other. When voltages are applied to the electrodes, an electric field is induced between the electrodes, and liquid crystal molecules are arranged by the electric field. The arrangement of the liquid crystal molecules varies depending on the electric field, and transmittance of light passing through the liquid crystal layer is controlled. Images can be displayed by controlling the light transmittance of the liquid crystal material.
Since an LCD device including thin film transistors as a switching element, referred to as an active matrix LCD (AM-LCD) device, has excellent characteristics of high resolution and displaying moving images, the AM-LCD device has been widely used.
The AM-LCD device includes an array substrate, a color filter substrate and a liquid crystal layer interposed therebetween. The array substrate may include a pixel electrode and thin film transistor, and the color filter substrate may include a color filter layer and a common electrode. The AM-LCD device is driven by an electric field between the pixel electrode and the common electrode to have excellent properties of transmittance and aperture ratio. However, since the AM-LCD device uses a vertical electric field that is perpendicular to the substrates, the AM-LCD device has poor viewing angles.
An in-plane switching (IPS) mode LCD device having a wide viewing angle property has been suggested and developed to resolve the above-mentioned limitations
Hereinafter, an IPS mode LCD according to the related art will be described with reference to accompanying drawings.
FIG. 1 is a cross-sectional view of schematically illustrating an IPS mode LCD device according to the related art.
In FIG. 1, an upper substrate 1 and a lower substrate 2 are spaced apart from each other, and a liquid crystal layer including liquid crystal molecules 3 is disposed between the substrates 1 and 2. A pixel electrode 4 and a common electrode 5 for driving the liquid crystal molecules 3 are formed on the lower substrate 2. When voltages are applied to the pixel electrode 4 and the common electrode 5, a horizontal electric field 6 parallel to the substrates 1 and 2 is generated between the pixel electrode 4 and the common electrode 5. The liquid crystal molecules 3 are moved and differently arranged by the horizontal electric field 6. The substrates 1 and 2 and the liquid crystal layer constitute a liquid crystal panel.
Upper and lower polarizers (not shown) are attached to upper and lower substrates 1 and 2 of the liquid crystal panel, respectively, and transmission axes of the upper and lower polarizers are perpendicular to each other. The liquid crystal molecules 3 change polarization state of light passing trough the lower polarizer according to their arrangement, and the light having the changed polarization state is selectively transmitted by the upper polarizer, thereby displaying an image.
In the IPS mode LCD device, the liquid crystal molecules are arranged according to the horizontal electric field, and thus viewing angles of the device are widened.
Moreover, the IPS mode LCD device has an advantage of low image distortion when it is used for a touch screen perceived by touch, and the IPS mode LCD device has been widely used for portable devices such as smart phones or tablet personal computers.
However, in the IPS mode LCD device according to the related art, there is no optical problem when it is seen at its front, and there is light leakage to cause an increase in black level of brightness and a decrease in contrast ratio when it is seen at its sides.
Accordingly, to compensate the optical properties at side viewing angles, one or more optical compensation films have been suggested and adopted between the polarizers and the liquid crystal panel.
Meanwhile, the potable devices are frequently used outside as well as inside. When a portable device is used outside, glaringness occurs due to sunlight reflected at its surface, and visibility is lowered. Therefore, a poly ethylene terephthalate (PET) layer may be formed on the upper polarizer over the liquid crystal panel to prevent the glaringness.
FIG. 2 is a view of schematically illustrating a polarizer according to the related art and corresponds to an upper polarizer.
In FIG. 2, the related art polarizer includes a polarizing film 12, first and second protective films 14 and 16, and a PET layer 18.
The polarizing film 12 is formed by stretching poly-vinyl alcohol (PVA), and n absorption axis of the polarizing film 12 is formed along a stretching direction. Light vibrating in a direction parallel to the absorption axis is absorbed by the polarizing film 12, and light vibrating in a direction perpendicular to the absorption axis is selectively transmitted by the polarizing film 12.
Poly-vinyl alcohol used for the polarizing film 12 has strong hydrophile property and weak moisture resistance. Thus, bonds between polymers in the polarizing film 12 are weakened by moisture, and the polarizing film 12 shrinks in the stretching direction. To prevent the shrinkage of the polarizing film 12, protective materials, which have low dimension changes by moisture, are attached to both sides of the polarizing film 12 and used for the first and second protective films 14 and 16, thereby restraining the shrinkage of the polarizing film 12.
The PET layer 18 is formed on the first protective film 14. The PET layer 18 prevents glaringness to improve visibility. In addition, the PET layer 18 increases surface hardness.
By the way, the PET layer 18 has phase retardation, and linearly-polarized light passing through the polarizing film 12 of the polarizer 10 is changed into elliptically-polarized light while passing through the PET layer 18.
In the meantime, sunglasses are used outside to prevent glaringness due to sunshine and to block UV rays. Polarized sunglasses are preferred because of vivid views and fine UV-blocking. However, when the portable device is viewed through the polarized sunglasses, color differences are caused according to viewing angles.
This will be explained with reference to FIG. 3.
FIG. 3 is a view of illustrating Poincare sphere showing polarization state of light passing through the polarizer of FIG. 2 and an absorption axis of polarized sunglasses.
In FIG. 3, the absorption axis A1 of the polarized sunglasses is disposed on the equator of Poincare sphere corresponding to linearly-polarized light, and the polarization state of light passing through the PET layer of the polarizer of FIG. 2 is disposed on the upper hemisphere of Poincare sphere corresponding to left-handed elliptically-polarized light.
Accordingly, when the LCD device including the polarizer of FIG. 2 is viewed through the polarized sunglasses, since the polarization state A2 of light passing through the polarizer of FIG. 2 does not coincide with the absorption axis A1 of the polarized sunglasses, degrees of polarization vary according to viewing angles, and the color differences are caused.
To solve the problem, additional films may be necessary. Particularly, an optical compensation film may be added to the polarizer to compensate the optical properties at side viewing angles. However, in this case, a thickness of the polarizer is increased by about 300 micrometers to about 400 micrometers.